As the hydraulic apparatus, there is a radial piston pump for sucking operation fluid and discharging the same to supply to a required portion, for example, and this is disclosed in a following patent document 1. In the radial piston pump disclosed in the patent document 1, a valve shaft having a suction port, a discharge port, a suction slot and a discharge slot is provided in a housing, a rotor is supported so as to be rotatable relative to the valve shaft, a cam ring is rotatably provided on an outer side of the rotor, a cylinder is radially provided on the rotor, and a piston is slidably mounted on the cylinder, thereby forming a pump chamber for selectively communicating with the suction slot and the discharge slot. Therefore, the operation fluid is sucked from the suction port through the suction slot into the pump chamber, and discharged through the discharge slot to the discharge port.
There is a case in which such radial piston pump is incorporated in the automatic transmission of the vehicle, the rotor of the radial piston pump is coupled to one of an input shaft and an output shaft, the cam ring of the radial piston pump is coupled to the other of the input shaft and the output shaft, to be used as an oil pump for discharging the oil according to difference in rotational speed between the input shaft and the output shaft, that is to say, a differential oil pump. In this case, at the time of positive driving such as when accelerating the vehicle, that is to say, when the rotational speed of the input shaft is higher than that of the output shaft and the input shaft relatively positively rotates, the radial piston pump can operate to discharge the operation fluid. However, at the time of being driven in which an engine brake acts on the vehicle, that is to say, when the rotational speed of the input shaft is lower than that of the output shaft and the input shaft relatively negatively rotates, a suction stroke and a discharge stroke of the operation fluid by the piston are inverted and the radial piston pump cannot discharge the operation fluid.
Therefore, in a following patent document 2, for example, a check valve is used to be able to appropriately discharge the operation oil at positive rotation and negative rotation of the input shaft. That is to say, in the oil pump of the patent document 2, a suction opening of the oil pump is connected to a liquid retaining unit through a first check valve and a discharge opening of the oil pump is connected to a discharge liquid requiring unit through a second check valve, and the discharge opening of the oil pump is connected to the liquid retaining unit through a third check valve and the suction opening of the oil pump is connected to the discharge liquid requiring unit through a fourth check valve, and it is arranged such that when the oil pump positively rotates, only the first and second check valves are opened, and when the oil pump negatively rotates, only the third and fourth check valves are opened. Therefore, the oil pump can discharge a predetermined flow amount regardless of a rotational direction of the motor.
In the conventional oil pump disclosed in the above-described patent document 2, a plurality of check valves are provided on the suction opening and the discharge opening of the oil pump, so that the oil can be discharged regardless of the rotational direction of the motor by opening the one group of check valves when the oil pump positively rotates and opening the other group of check valves when the oil pump negatively rotates. However, the check valve is for releasing a pressure of the oil when this is higher than a predetermined pressure, so that pressure loss of the oil is large and cavitation and operational inadequacy of the piston (cam nose jumping phenomenon) easily occur when sucking the oil, and further, there is a problem that mechanical efficiency deteriorates.
In addition, although the differential oil pump is for sucking to discharge the oil as the operation fluid according to the rotational speeds of two rotating bodies, it is desirable that this is used as the motor for obtaining power by rotating one rotating body by supplying the oil while restraining the other rotating body. However, in the above-described conventional oil pump, a plurality of check valves are provided on the suction opening and the discharge opening of the oil pump, so that it is difficult to allow the same to serve as the motor.
Patent Document 1: Japanese Patent Application Laid-open No. 02-108866
Patent Document 2: Japanese Patent Application Laid-open No. 09-303256